Gas Chromatography Mass Spectrometry (GCMS)

Transcription

1 Gas Chromatography Mass Spectrometry (GCMS)

2 Organics Chlorinated pesticides DDT Chlordanes Polychlorinated biphenyls (PCBs) Polyaromatic hydrocarbons (PAHs)

3 DDT (Dichlorodiphenyl-trichloroethane) Used as an insecticide Control malaria and typhus during WW II

4 DDT (Dichlorodiphenyl-trichloroethane) Used as an insecticide Control malaria and typhus during WW II <80% 4,4 -DDT, < 20% 2,4-DDT, and < 4% 4,4 - DDD Breaks down into 4,4 -DDE, 2,4 -DDE and 2,4-DDD Used as an agricultural pesticide Banned in 1972 in the US Causes egg shell thinning Southern CA- world s largest DDT manufactor Montrose Chemical Corporation 4,4 -DDT 4,4 -DDD 4,4 -DDE

5 PCBs 209 congeners Often referred as Aroclor i.e.- Aroclor 1260 Used for hydraulic fluids, insulators, carbonless papers Banned in 1976 in US

6 Polyaromatic hydrocarbons (PAHs) Occur in oil, coal and tar Benzo [a] pyrene- 1 st chemical carcinogen discovered

7 Analytical Procedure Sample Preparation Extraction Cleanup Quantification Quality Assurance/Quality Control

8 Sample Preparation Involves modifying the physical form of the sample so that the analytes can be extracted efficiently Homogenize sample

9 Extraction What is the purpose? Used when we want to separate substances How do we separate substances? Use a solvent in which the desired substance dissolves in and the undesired substance does not dissolve in Example: coffee grounds Insoluble matrix: coffee grounds Soluble analytes: coffee flavor Analytes can be leached out in a suitable solvent (water)

10 Soxhlet Extractor Cellulose thimble: g of sample inside Condenser Extractor 200 ml 3:1 DCM:acetone Boiling flask

11 Soxhlet Extractor 2) Condensed drops of solvent fall into thimble, dissolving analytes from the matrix 1) Boiling solvent vapors rise up

12 3) When smaller side-arm fills to overflowing, siphoning action is initiated Soxhlet Extractor

13 4) Solvent (containing the dissolved analytes) is siphoned into the flask Soxhlet Extractor

14 5) Residual solvent drains out of cup, as fresh solvent continues to fall Soxhlet Extractor

15 Concentrate sample Called rotovaping From ~200 ml to ~ 1 ml Lipid determination (infaunal samples only) Split the sample 50:50 Solvent exchange: DCM to hexane Add ~ 2 ml hexane and concentrate again to ~ 1mL

16 Extract cleanup Column chromatography: silica gel and alumina cleanup Aim: to remove compounds that may interfere with analyte analysis Principle: Concentrated sample is deposited onto the column Column is washed with the appropriate solvents 30 ml hexane 15 ml 30% DCM/Hexane 15 ml DCM Analytes are eluted with the solvents whereas the interfering compounds remain on the column Concentrate sample From ~60 ml to ~ 1 ml

17 Quality Assurance/Quality Control (QA/QC) Quality Assurance vs. Quality Control An overall management plan to guarantee the integrity of the data Series of analytical measurements used to assess the quality of the analytical data

18 Precision Are results reproducible? vs. Accuracy Are results accurate?

19 Precision Are results reproducible? Field duplicates vs. Accuracy Are results accurate?

20 Duplicate error, n = relative % difference tpcb tpcb: analyzed 6 mo apart tddt tddt: analyzed 6 mo apart concentration (μg/g lw -1 )

21 Precision Are results reproducible? Field duplicate vs. Accuracy Are results accurate? Surrogates Blanks Matrix spike (MS) Do in duplicate: Matrix spike duplicate (MSD) Standard reference material (SRM)

22 Surrogates Monitors extraction efficiency Most common in organic methods Similar to analytes you are analyzing Added to ALL field samples and QAQC samples Added prior to extraction Surrogate recovery criteria?

23 Method Blank Blanks, Blanks, Blanks Clean sample (solvent only) subjected to ALL sample prep Instrument Blanks Clean sample (e.g.- hexane) used to determine instrument contamination Field Reagent Blanks Prepared in the lab, taken to the field Opened at the sampling site, exposed to sampling equipment, returned to lab Trip Blanks Prepared in the lab, taken to the field Not opened, returned to the lab

24 Matrix spikes (MS) Sample spiked with a known amount of ALL analytes Subjected to ALL sample prep and analytical procedures Determines the effect of the matrix on analyte recovery Done in duplicate Laboratory Control Spikes (Blank spike) All analytes spiked into solvent Subjected to ALL sample prep and analytical procedures Done in duplicate

25 Standard Reference Material (SRM) Homogeneous material where analyte values have been well established to validate lab and analytical methods Use similar matrix to samples Lake Michigan Fish Tissue (SRM 1947) Sediments (1941b)